1. Field of the Invention
This invention relates to production of an inner grooved metallic tube, and more particularly to an inner grooving process for a metallic tube which is suitable for a heat transfer tube of a heat exchanger for an air conditioner, a refrigerator or the like.
2. Description of the Prior Art
Various metallic tubes which have inner grooves of various shapes formed therein are desired as metallic tubes of copper, aluminum and similar materials which are used for heat transfer tubes of heat exchangers for an air conditioner, a refrigerator or the like.
Conventionally, as a process of producing such an inner grooved metallic tube, there is a rolling drawing process as one of so-called tube reducing type grooving processes wherein grooves are formed on an inner wall of a metallic tube to be worked while the metallic tube is being reduced in diameter as disclosed, for example, in Japanese Patent Laid-Open No. 54-37059 and Japanese Patent Laid-Open No. 55-103215.
According to the rolling drawing process, as illustrated in FIG. 3, while a metallic tube 21 is gripped at an end thereof and is drawn by a chuck, it is pressed from the outside by means of a hole die 22 and a floating plug 23 to reduce the diameter thereof, and then an inner wall of the metallic tube 21 is pressed against a grooved plug 25 mounted upstream in the tube 21 by means of rolling rollers or balls 24 disposed around an outer periphery of the metallic tube 21 in order to form grooves 26 on the inner wall of the tube 21.
According to the process, however, since the revolving rolling rollers or balls 24 are pressed against the metallic tube 21 at a location where the grooved plug 25 is located to reduce the inner and outer diameters of the metallic tube 21 to form grooves on the metallic tube 21, if the drawing speed of the metallic tube 21 is excessively high, some portions of the metallic tube 21 may not be contacted by the rolling rolls or balls 24 and pressed portions of the metallic tube 21 may be discontinuous from each other, causing grooves to be formed intermittenttly, or else a reactive contraction force may act upon a portion of the metallic tube 21 which has just passed the pressing location of the rolling rollers or balls 24 so that such a floating phenomenon as illustrated in FIG. 4 may appear to cause the metallic tube 21 to be distorted. In this manner, there is a high possibility that displacement in phase of groove pitch and various interferences including a fillet interference, a trochoid interference and an involute interference may appear in combination to cause internal defects in the metallic tube 21. Further, since the metallic tube 21 having an inner diameter greater than an outer diameter of the grooved plug 25 is reduced in diameter and rolled by the rolling rollers or balls 24 so as to force the metallic tube 21 into the grooves of the grooved plug 25 to groove the metallic tube 21, a high degree of deformation is applied to the metallic tube 21. Accordingly, there is a problem in that metal flows may not be uniform at grooves or ridges of the metallic tube 21 so that defects may be produced at the slopes or bottoms of the grooves. Accordingly, where such a metallic tube is employed as a heat transfer tube, there are problems of reduction in pressure resisting strength, reduction in vibration fatigue strength, generation of metal powder, occurrence of a slow leak of a refrigerant caused by occurrence of a defect in brazing and so on.
In order to resolve such disadvantages of the rolling drawing process of such a tube reducing type as described above, a so-called tube expanding rolling process has been proposed wherein an inner wall of a metallic tube is grooved while the metallic tube is being expanded as illustrated in FIG. 5 (Japanese Patent Laid-Open No. 61-266121). According to the tube expanding process, at first a metallic tube 21 is reduced in diameter by means of a drawing die 29 and a floating plug 23 in a similar manner as in the tube reducing type rolling drawing process described above, and then it is grooved while it is being expanded by means of a grooved plug 28 which has an outer diameter greater than the inner diameter of the metallic tube 21.
However, while the tube expanding type rolling process can eliminate the disadvantages of the tube reducing type rolling process described above, it has a problem in a method of inserting the grooved plug 28, having a greater outer diameter than the inner diameter of the metallic tube 21, into the metallic tube 21 in advance.
In particular, where such a hole die as employed in the tube reducing type rolling process is employed as the drawing die 29, it is impossible to insert the grooved plug into the metallic tube 21 because the diameter of the exit hole thereof is fixed. Accordingly, in order to insert the grooved plug into the metallic tube 21, it is necessary either to construct the drawing die 29 such that the inner diameter of the exit hole thereof may be changed mechanically to a size sufficient to allow the grooved plug 28 to be inserted into the exit hole thereof or to cut an end portion of the metallic tube 21 into two arcuate portions over a predetermined length so as to allow insertion into the end portion of the metallic tube 21 of the grooved plug 28 which is to be subsequently connected to the floating plug 23. Accordingly, in the case of the former, the equipment is complicated in structure, while in the case of the latter, operability is significantly deteriorated.